Support for differential inner core superrotation from earthquakes in Alaska recorded at South Pole station

Abstract

Observational evidence for differential inner core rotation has recently been reported. The most convincing evidence has been restricted to one ray path from earthquakes in the South Sandwich Islands to stations in Alaska. Here we show that the differential travel times between PKP(DF) and PKP(BC) for a new ray path from earthquakes in southern Alaska to the station at the South Pole have increased by about 0.6 s over 37 years, providing support for a differential inner core rotation. We apply a newly developed technique to invert simultaneously for the inner core structure and the rotation rate. The rotation rate determined from the new pathway is about 0.6°/yr faster than the mantle, compatible with the estimates of the rate from earthquakes in the South Sandwich Islands to stations in Alaska. We also address some criticisms concerning the reported detection of the differential inner core rotation, including biases from event mislocations, biases from mantle heterogeneity, and uncertainty in the tilt of the inner core anisotropy axis. We show that although these potential biases would affect the determination of the rotation rate, they are unlikely to account for the observed time dependence in the BC-DF times. We found no evidence for a correlation between BC-DF residuals and event magnitudes as suggested previously. Copyright 2000 by the American Geophysical Union.